Ink jet printing technology provides printers with variable capability. There are three main ink jet technologies: thermal, i.e. bubble jet, piezoelectric, and continuous. In each, tiny droplets of ink are fired (i.e., sprayed) onto a page. In a thermal jet printer, a heat source vaporizes ink to create a bubble. The expanding bubble causes a droplet to form, and the droplet is ejected from the print head. Piezoelectric technology uses a piezo crystal located at the back of an ink reservoir. Alternating electric potentials are used to cause vibrations in the crystal. The back and forth motion of the crystal is able to draw in enough ink for one droplet and eject that ink onto the paper. In a continuous ink jet system, the nozzles are continuously firing and an electrode associated with each nozzle deflects the drops to a gutter for collection when the nozzle is not to print. When a nozzle is to print the electrode is deactivated and the drop will pass to the substrate.
The quality of high speed color ink jet printing is generally lower than that of offset lithography and gravure. Furthermore, the speed of the fastest ink jet printer is typically much slower than a lithographic or gravure press. Traditional aqueous-based ink jet printing is also plagued by the effect of placing a water-based ink on paper. Using a water-based ink may saturate the paper and may lead to wrinkling and distortion of the printed paper web, and the web may also be easily damaged by inadvertent exposure to moisture. In order to control these phenomena, ink jet printers use certain specialized papers or coatings. These papers can often be much more expensive than a traditional web paper used for commercial print.
Furthermore, when ink jet technology is used for color printing, ink coverage and water saturation may be increased. This is due to the four color process that is used to generate color images. Four color processing involves depositing cyan, magenta, yellow and black (i.e., CMYK) ink in varying amounts to make a specific process color on the page. Thus, some portions of the page may have as many as four layers of ink if all four colors are necessary to produce the desired process color. As a result of the total printed drop volume of the various inks needed to print the desired process color, the dots printed by an ink jet printer may spread and produce a fuzzy image. Control of the drop spread or growth is achieved by careful balance of the formulation components to optimize the drop spread or growth to give the desired printed drop overlap without fuzziness or intercolor ink mixing. Still further, inks used in ink jet printers tend to be more expensive as compared to inks used in traditional lithography or gravure printing. However, careful control of dot drop growth and ink interaction with the paper substrate through precise formulation control can lower the overall cost associated with using inkjet type inks which allow for the use of the highly advantageous variable nature of the inkjet printing technology without sacrifice in final print quality.
In some cases, it is therefore appropriate to use high speed ink jet printing rather than lithography or gravure. Under such circumstances, it is desirable for the images on the web to have long term durability. Long term durability refers to the ability of the ink to remain on the final printed product and resist potential damage caused by post-printing water exposure, highlighting markers, and other mechanical abrasions to the printed document. Additionally, paper types are grouped into both uncoated and coated papers that have varying degrees of porosity to water-based inkjet inks, with the coated papers exhibiting lower porosity levels and slower absorption rates of the water-based inkjet inks. The result is that long term durability of inkjet images on coated papers tend to be of a reduced quality in comparison to uncoated inkjet printed papers in terms of the properties listed previously.
One final challenge is the characteristic of short term durability that is important in the immediate printed paper movement or transport through the printing system. Paper folding and forming operations through the printing system can damage the freshly printed inkjet image due to shear forces and contact with various print system hardware. For example during the printing process there is a tendency for the ink to be disturbed by shear stresses as the web moves through subsequent cylinders and/or other elements of the high speed printing press. Formulation designs that allow for quick ink setting on the paper sheet and immediate resistance to smearing or offsetting in the printing system will be advantageous.